Collision with an asteroid is one of the least of the dangers we now face. The last collision was 70 million years ago. Nuclear war, climate change, and pollution, are now much greater threats. Of course it would be impossible to move the population of the Earth to a new planet, but if we can establish independent self-sustaining colonies in space, it would ensure the survival of the human race, in the event of a disaster on Earth. Leaving Earth will also give us a new perspective, and cause us to look outwards rather than inwards. It would also unite us to face a common challenge.